This invention relates generally to an illumination system for illuminating the surface to be illuminated, by use of light from plural light sources. More specifically, the invention concerns a method and a structure for controlling the light quantity of an illumination system to be used in a process for exposing a workpiece such as a monocrystal substrate for a semiconductor wafer or a glass substrate for a liquid crystal display (LCD), for example, as well as an illumination system and an exposure apparatus having such structure. The present invention is suitably applicable to a control system for an illumination system to be used in a projection exposure apparatus, for example, in which a glass substrate for an LCD is exposed by a scan method in a photolithographic process by use of plural light sources. However, the applicability of the present invention is not limited to such exposure apparatus, and it can be applied widely to photolithography, projection inspection, projectors and so on.
The scan method mentioned above is an exposure method in which an illumination optical system for illuminating a mark or a reticle (hereinafter, mask) and a projection optical system disposed between the mask and a workpiece, such as a wafer being coated with a resist, are used and in which a portion of a pattern is projected by the projection optical system onto the resist while the mask and the workpiece are scanningly moved in synchronism with each other, relative to the projection optical system, whereby the whole pattern is transferred to the workpiece.
Generally, projection exposure apparatuses have an illumination optical system for illuminating a mask, and a projection optical system disposed between the mask and a workpiece. In order to obtain a uniform illumination region, in such illumination optical system, the light from a light source is introduced into an optical integrator (hereinafter, referred also to xe2x80x9cintegratorxe2x80x9d) such as a fly""s eye lens, and, while using the light exit surface of the integrator as a secondary light source surface, a condenser lens illuminates the mask surface.
In projection exposure apparatuses, it is desired to increase the light quantity (or an exposure illuminance) of an illumination optical system to thereby improve the throughput. A generally adopted method therefore is enlargement of the output power of a high-pressure halogen lamp which is a light source of the illumination optical system. In the first half 1980s, those lamps of a few hundred watts (W) to 2.0 kW were used. In the first half of 1990s, lamps of 3 kW were developed and, in the second half of 1990s, lamps of 5 kW were developed. Development of higher power lamps still continues.
However, since use of a higher power lamp causes increases in size and cost of the illumination system, proposals have been made recently to use plural light sources and to combine light fluxes emitted from the light sources. For example, Japanese Laid-Open Patent Application, Laid-Open No. 349710/1994 (U.S. Pat. No. 5,815,248) and Japanese Laid-Open Patent Application, Laid-Open No. 135133/1995 (U.S. Pat. No. 5,815,248) show an exposure apparatus of san projection type, having two light sources, comprising a tilted lamp and an elliptical mirror. In these documents, light fluxes emitted from the light sources are introduced into a fly""s eye lens and are combined with each other there.
Where the light quantities of these light sources are controlled and if the structure is arranged so that, when any one of the light sources is out of order, the light quantity is compensated by the output of the other light source, it applies a large burden. In Japanese Laid-Open Patent Application, Laid-Open No. 349710/1994, there is a checking sensor for exposure control which is disposed at the back of each elliptical mirror. The checking sensor comprises a light receiving element for receiving leakage light from the elliptical mirror, and a timer for integrating the time during a period in which a photoelectrically converted signal from the light receiving element is higher than a predetermined level. The checking sensor functions to control the replacement timing of the light source and also to discriminate the number of light sources being turned on. The exposure control is made thereby so that, when a single light source is turned on, the scan speed is made a half of that where two light source are turned on. In Japanese Laid-Open Patent Application, Laid-Open No. 135133/1995, a half mirror is disposed obliquely, with respect to the light path, before the light fluxes emitted from the lamps are collected by an elliptical mirror. A light receiving element is disposed at the point of convergence of the light divided by the half mirror. The result of detection thereby (i.e., monitor signal) is used to control the light quantities of the lamps.
However, these conventional illumination systems using plural light sources involves a problem that high precision light quantity control is unattainable. The image quality of a pattern transferred to a workpiece is largely influenced by the performance of the illumination system, for example, the illuminance distribution upon the mask surface or the wafer surface. For this reason, an exposure apparatus having such illumination system as described above is unable to accomplish high precision exposure amount control. As a result, the pattern transfer to a resist is insufficient, and production of high quality semiconductor wafers, LCDs, thin film magnetic heads, etc., is unattainable. Particularly, where a light source (halogen lamp) having a lifetime of about 1,000 hours is replaced by a new halogen lamp, during a few ten hours in the initial stage of operation, consumption of lamp electrodes is large and also the luminance varies largely. During such period in which the lamp electrode consumption is large, it has been found that there is an error of a few percents produced between the light quantity as can be measured by the light receiving element of Japanese Laid-Open Patent Application, Laid-Open No. 135133/1995 and the quantity of light projected to the mask surface where uniform illumination is performed.
According to the investigations about this phenomenon, made by the inventors of the subject application, it has been found that, in the structure of this document, the sensor is disposed near the light source and, consequently, the light quantity of each light source is not controlled on the basis of the light quantity which the workpiece actually receives, in spite of that for precise exposure amount control the light quantity which a workpiece actually receives should be detected and the light quantities (or illuminances) of the plural light sources should be feedback-controlled on the basis of the detection result (i.e., monitor signal).
It is accordingly an object of the present invention to provide a unique and useful illumination system and an exposure apparatus having such illumination system.
It is another object of the present invention to provide an illumination system in which plural light sources are used and light fluxes emitted from plural light sources are combined for illumination, and in which the light quantities of these light sources are measured very precisely.
It is a further object of the present invention to provide an illumination control system and an illumination control method for such illumination system described above.
It is a further object of the present invention to produce high quality devices such as semiconductors, LDCs, CCDs, thin film magnetic heads, etc., on the basis of high precision exposure amount control using such illumination system as described above.
In accordance with an aspect of the present invention, there is provided an illumination system, comprising: a first optical system for combining plural light fluxes from plural light sources and for projecting the plural light fluxes to a surface to be illuminated; a second optical system for separating a portion of one of the combined plural light fluxes; and a first detecting system for detecting the light quantity of the portion separated by said second optical system.
In accordance with another aspect of the present invention, there is provided an illumination system wherein the surface to be illuminated and a light receiving surface of said first detecting system are placed in an optically conjugate relationship.
In accordance with a further aspect of the present invention, there is provided an exposure apparatus for illuminating a reticle or a mask by use of an illumination system as recited above, and for exposing a workpiece with a pattern formed on the surface being illuminated. In this exposure apparatus, there may be a third optical system for projecting the pattern of the reticle or the mask onto a workpiece, wherein the workpiece comprises a glass plate for an LCD, and wherein the pattern is transferred to the workpiece through a scan of the reticle or the mask and the workpiece.
In accordance with a yet further aspect of the present invention, there is provided a control system, comprising: an optical system for separating a portion of one of plural light fluxes emitted from plural light sources and being separatably combined; detecting means for detecting the light quantity of the portion of the one light flux separated; and control means for controlling the light quantity of one of the plural light sources corresponding to the one light flux, on the basis of the detection by said detecting means. With this control system, the light quantity of the corresponding light source can be controlled very precisely on the basis of the light quantity of one light flux as separated by the optical system.
In accordance with a still further aspect of the present invention, there is provided an illumination method, comprising the steps of: discriminating whether a first light quantity, of a first light flux separated from plural light fluxes which are emitted from plural light sources, including a first light source for emitting the first light flux, and which are being separatably combined, is within a first target range; and controlling the output of said first light source, when it is concluded that the first light quantity is outside the first target range, so that the first light quantity is kept within the first target range. With this method, the first light flux is separated from the combination with the second light flux and is detected. The output of the first light source is controlled on the basis of the detection.
In accordance with a still further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a workpiece by use of an illumination light flux controlled in accordance with an illumination method as recited above; and performing a predetermined process to the exposed workpiece. The device manufacturing method can provide the same function as of the illumination control method described above. The scope of the method extends to intermediate and final products, that is, devices which may be, for example, semiconductor chips (such as LSI or VLSI), CCDs, LCDs, magnetic sensors, thin film magnetic heads, etc.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.